In recent years, as a storage device mounted on a storage unit, a storage device, which is accessible at higher speed than a HDD (Hard Disk Drive), represented by an SSD (Solid State Drive) in which a NAND-type flash memory (FM) is used as a storage medium, has been spread. Further, a storage device including a semiconductor storage medium, which is accessible at higher speed than a flash memory, such as a ReRAM (Resistance Random Access Memory) or a PCM (Phase Change Memory) is developed for practical use.
In most of storage units, availability of a system is increased using a RAID (Redundant Array of Inexpensive/Independent Disks) technique. The RAID technique is a technique in which a controller (a storage controller) of a storage unit generates redundant data such as parities from write data received from a host apparatus such as a host computer and distributedly stores the write data and the parities respectively in different storage devices. By adopting the RAID technique, even when a failure occurs in a part of the storage devices and data cannot be read from the storage device, the data can be restored using information stored in the other storage devices.
For example, PTL 1 discloses a technique in which, when one of storage devices breaks down, a storage controller reads data and parities from the storage devices other than the broken-down storage device and the storage controller restores the data and writes the restored data in a new storage device.